You should note when making the cryptography code that there are some irregularities with OpenSSL which is what bitcoin-qt uses and your code should be compatible with these irregularities. Discussed here: http://bitcoinstats.com/irc/bitcoin-dev/logs/2012/06/05/2#l3670669 It's possible that if you have a client that does not take care of these OpenSSL irregularities then particular public keys and maybe signatures in transactions would be accepted by OpenSSL but not your code. This means bitcoin-qt would accept the transactions as valid but your code wont.